This invention relates to a process for thermal cracking of a heavy hydrocarbon oil (hereinafter abbreviated as heavy oil) to obtain primarily light hydrocarbons (hereinafter abbreviated as light oils) which are liquid at room temperature. More particularly, the present invention relates to an improvement of a process, which comprises carrying out a step of thermal cracking in which a heavy oil is contacted with fine particles of a porous material fluidized with a steam-containing gas and the step of gasification in which the coke deposited on the fine particles from the thermal cracking step is removed while the fine particles are fluidized with a molecular oxygen-containing gas or a steam-containing gas, while the fine particles are circulated between both of these steps.
Some of the present inventors have previously discovered that, in thermal cracking of a heavy oil in contact with a fluidized-bed of heated particles can be practiced under good fluidized state of the bed and with good efficiency when use is made of fine particles comprising particles having a weight average diameter of 0.04 to 0.12 mm and 5 to 50 wt. % of particles with a diameter of 0.044 mm or less, and named this process as Fluid Thermal Cracking (Japanese Laid-open Patent Publication No. 10587/1981).
They have also disclosed that when use is made of fine particles which have a pore volume of 0.1 to 1.5 m.sup.3 /g, a specific surface area of 50 to 1500 m.sup.2 /g and a weight average diameter of 0.025 to 0.25 mm and are thermally stable, thermal cracking can be practiced at still improved efficiency. They found that absorption of liquid heavy oil by the porous material promotes the thermal cracking reaction or inhibits the formation of highly carbonaceous solids (hereinafter abbreviated as coke), and they called this "the capacitance effect" (see Japanese Laid-open Patent Publication No. 18783/1982).
Further, they have disclosed that in a similar process, comprising the step of thermal cracking of a heavy oil and the step of removing by gasification the coke deposited on the fine particles of a porous material, withdrawn from the thermal cracking step, wherein the gasification is accomplished by cotacting the fine particles with an oxygen-containing gas while the fine particles are circulated between the both steps, an effective embodiment has been shown, in which the fluidized-beds formed in the both steps are arranged adjacent to both sides of a thermally conductive partition wall (see Japanese Laid-open Patent Publication No. 158291/1982).